The exploration and development of oil and gas resources in China has been oriented to oil and gas resources in oceans and deep strata and unconventional oil and gas resources. Complex oil and gas wells that have special structures, such as deep wells and extra-deep wells, etc., have been widely applied, which has raised higher requirements for oil and gas well cementing techniques. In high-temperature environments, the settlement instability problem of cement slurry becomes more prominent, has strong impacts on the quality of cementing work and threatens the safety of the work.
The settlement stability of cement is one of important indexes for evaluating the performance of the cement as well as one of important factors that have influence on the quality of cementing work. The poor settlement stability of cement slurry is mainly resulted from two causes: one cause is the high-temperature thinning and dispersion effects of the additives and admixtures of the cement; the other cause is the intensified Brownian movement of the molecules in the particles of the cement slurry at high temperature, which destroys the viscous force in the slurry and thins the slurry. Wherein, slurry thinning resulted from high-temperature sensitivity of cement slurry is the principal cause for the poor settlement stability of cement slurry. Once the cement slurry becomes instable during settlement, the solid particles may settle and free water may precipitate, causing bridge plugging or forming oil, gas, and water channeling paths; thus, the non-uniformity of the hardened cement formed in that process will be more prominent and eventually result in degraded quality of failure of cementing.
To alleviate the problem of settlement instability of high-temperature cementing cement slurry, measures such as decreasing the water-cement ratio of the oil well cement slurry appropriately, performing deep-processing of the particles of admixtures, and optimizing the grading of the cement particles, etc., may be taken. Though those measures can solve the problem of settlement instability of cement slurry to some extent, they still have the following drawbacks: (1) the initial consistency of the slurry may be excessively high, resulting in increased difficulties in slurry pumping; (2) the processing cost is high, and the process is complex; (3) the stability of the cement slurry may be improved in low temperature cementing environments, but the effect is not ideal in high-temperature environments, such as deep wells and extra-deep wells, etc. Therefore, presently, settlement stabilizers are widely used to alleviate the problem of settlement instability. The settlement stabilizers include inorganic materials and high molecular polymers, and have advantages including low cost, simple process, and excellent thickening effect, etc. Wherein, settlement stabilizers made of inorganic materials are less susceptible to temperature and have an advantage that they are not degraded or diluted at high temperature; however, the side effects incurred by the thickening effect are obvious, and the initial consistency of the slurry is high, resulting in difficulties in cement injection and pumping and increased difficulties in cementing operations; though settlement stabilizers made of natural high molecular polymers have advantages including high viscosity, wide sources, and low cost, they will be degraded severely as the temperature is increased; consequently, the viscosity is decreased obviously, and the settlement inhibition ability of the system is severely decreased; though settlement stabilizers made of synthetic high molecular polymers have stable performance, are easy to regulate and control, and can effectively improve the viscosity of the slurry and remarkably improve the settlement stability of the cement slurry, they depend on the temperature closely, and may be hydrolyzed and thinned during shearing at high temperature; consequently, the consistency of the slurry is obviously decreased, and the settlement inhibition capability is severely degraded.
Zhang Hao, Li Houming (Zhang Hao, Cementing with stable suspending agent SS-10L study application [J]. Science technology and engineering, 2014), developed an oil well cementing using suspension stabilizer, by Welan gum as an organic suspension components, mineral B as inorganic suspended components and the composition of surfactant C, the experimental results and the field application shows that the added amount of suspension stabilizer is extremely low, good compatibility with admixture, cost-effective, but poor heat resistance ability, makes the slurry thickening in low temperature and high temperature is thinning. Xia Chun (Xia Chun, the mechanism research of ACS New grouting stabilizer [J]. Journal of civil engineering, 2005, 38 (6): 89-91.), developed ACS slurry stabilizer by polyacrylamide is with acrylic acid graft copolymer and compounded inorganic chemical materials, and other components of the, belongs to the polymer surface active agent, the results show that the ACS slurry stabilizer can obviously improve the cement strength, low water cement ratio, the enhancement effect is significant, but the effect is not obvious when high water cement ratio.
The patent document CN105176505A has disclosed a cement slurry stabilizer for cementing, which comprises 54-89pbw quartz sand, 2-20pbw Welan gum, 1-8pbw xanthan gum, 3-10pbw polyvinyl alcohol, and 3-8pbw grafted high molecular polymer. The stabilizer not only can improve the stability of cement slurry, but also have no adverse effect to other properties of the cement slurry in the application process. In addition, the stabilizer is beneficial for the compression strength of the set cement. However, the cement stabilizer can't be applied well to high-temperature cementing, because it doesn't have a thermoviscosifying effect and may be diluted at high temperature.
The patent document CN107162512A has disclosed a temperature-resistant cement slurry system for extra-deep wells, which comprises cement, high-temperature filtrate reducer, silica sand, high-temperature setting retarder, high-temperature stabilizer and disperser. By adding temperature-resistant admixtures, the cement slurry system solves the problem of unstable performance of the cement slurry at high temperature; however, pumping of the slurry is more difficult because the initial consistency of the slurry is excessively high.
The patent document CN107892514A has disclosed settlement-inhibiting cement slurry that is stable at high temperature, wherein, gaseous silicon dioxide and photovoltaic silicon are added into the cement slurry after a series of high-temperature admixtures are added. Therefore, the cement slurry has advantages including adjustable thickening time and better inhibition ability against degradation at high temperature; however, the compression strength of the hardened cement is affected to some degree, and the quality of cementing work is affected adversely.
In view of the drawbacks of existing thickeners and stabilizers, such as increased difficulties in slurry pumping incurred by excessively high initial consistency of slurry in low temperature environments, and degraded quality of cementing work incurred by thinning of the cement slurry at high temperature, etc., there is an urgent need for a novel synthetic polymer to attain the objective of settlement control of high-temperature cement slurry. The novel polymer shall have a characteristic that it doesn't thicken at low temperature and thickens as the downhole temperature is increased. At present, N-alkylated acrylamide polymers, which are conventional temperature-sensitive polymers discovered at the earliest, are applied widely. The conventional temperature-sensitive polymer N-alkylated acrylamide polymers realize temperature-sensitive thickening by introducing an amphiphilic temperature-sensitive monomer. When the temperature is lower than the lower critical solution temperature (LCST) (i.e., the turbidity point), N-alkylated acrylamide polymers mainly exhibit hydrophilicity and can be dissolved uniformly in water; in that state, the apparent viscosity of the water solution is low; when the temperature exceeds the lower critical solution temperature (LSCT), the hydrophobicity of N-alkylated acrylamide polymers is enhanced, the hydrogen bonds between amido groups in the molecular chains and water molecules are broken, and the hydrophobic groups on the molecular chains start to contract and accumulate as the temperature is increased; thus, the polymer molecules start to accumulate and precipitate from the liquid phase, and thereby the viscosity of the solution is increased significantly. However, since the hydrophobicity of N-alkylated acrylamide temperature-sensitive polymers is weak, and the thickening temperature range is 20-50° C., N-alkylated acrylamide polymers can't meet the requirements of high-temperature cementing, and are unsuitable for use as settlement stabilizers for high-temperature cement slurry.
Up to now, there is no satisfactory solution to the problem of settlement instability of high-temperature cement slurry yet. Especially, as the temperature of the strata is increased, the settlement instability of the oil well cement slurry becomes more prominent. Therefore, further invention and development are required to synthesize a novel oil well cement settlement stabilizer.